Remove GL image support from SkSL

This was added to Ganesh briefly in 2016 for an experiment, then
reverted, and never used.

Change-Id: I65849aff53acc14f9adc7585398fd09363a4ed65
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/374376
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
12 files changed